There are currently no international norms which define a method for characterizing photovoltaic solar cells for indoor applications. The current standard test conditions are not relevant indoors. By performing efficiency simulations based on the quantum efficiency of typical solar cells and the light spectra of typical artificial light sources, we are able to propose the first step for developing a standard by determining which light sources are relevant for indoor PV characterization and which are not or are redundant. Our simulations lead us to conclude that indoor light sources can be divided into three different categories. For the characterization of photovoltaic solar cells in indoor environments, we propose that solar cells be measured under one light source from each group.
In order for organic bulk heterojunction solar cells to compete with the traditional inorganic cells, power conversion efficiencies of more than 10% are desirable. Nowadays, efficiencies up to 5% are reached and the question about the limits for the attainable efficiency of organic cells arises. In this paper, we study the efficiency potential of organic bulk heterojunction solar cells. We make realistic assumptions to predict efficiencies obtainable in the near future, and calculate the upper-limit. We study the influence of the difference between the lowest unoccupied molecular orbital (LUMO)-energy levels of donor and acceptor, and the absorption window on the efficiency. Ideal material characteristics are obtained from these calculations, giving an idea how the ideal organic solar cell should look like. The calculations show that nowadays an efficiency of 5Á8% for the single junction bulk heterojunction solar cell should be possible. Considering parameters which are credible to be achieved in the future, an organic solar cell of 15Á2% is in reach, with an optimal bandgap of 1Á5 eV for the absorber. We also consider the situation where both the nand p-type materials are absorbers. All calculations are not only done for a single junction cell, but also for tandem solar cells. For a tandem structure of organic cells, we find in a realistic scenario a maximum attainable efficiency of 10Á1% and an efficiency of 23Á2% in an optimistic scenario with optimal bandgaps E g1 ¼ 1Á7 eV and E g2 ¼ 1Á1 eV.
Organic solar cells have narrow absorption windows, compared to the absorption band of inorganic semiconductors. A possible way to capture a wider band of the solar spectrum—and thus increasing the power conversion efficiency—is using more solar cells with different bandgaps in a row, i.e., a multi-junction solar cell. We calculate the ideal material characteristics (bandgap combinations and absorption windows) for an organic tandem and triple-junction solar cell, as well as their acceptable range. In this way, we give guidelines to organic material designers.
Wireless power transfer using a magnetic field through inductive coupling is steadily entering the market in a broad range of applications. However, for certain applications, capacitive wireless power transfer using electric coupling might be preferable. In order to obtain a maximum power transfer efficiency, an optimal compensation network must be designed at the input and output ports of the capacitive wireless link. In this work, the conjugate image theory is applied to determine this optimal network as a function of the characteristics of the capacitive wireless link, as well for the series as for the parallel topology. The results are compared with the inductive power transfer system. Introduction of a new concept, the coupling function, enables the description of the compensation network of both an inductive and a capacitive system in two elegant equations, valid for the series and the parallel topology. This approach allows better understanding of the fundamentals of the wireless power transfer link, necessary for the design of an efficient system.
Wireless power transfer allows the transfer of energy from a transmitter to a receiver without electrical connections. Compared to galvanic charging, it displays several advantages, including improved user experience, higher durability and better mobility. As a result, both consumer and industrial markets for wireless charging are growing rapidly. The main market share of wireless power is based on the principle of inductive power transfer, a technology based on coupled coils that transfer energy via varying magnetic fields. However, inductive charging has some disadvantages, such as high cost, heat dissipation, and bulky inductors. A promising alternative is capacitive wireless power transfer that utilizes a varying electric field as medium to transfer energy. Its wireless link consists of conductive plates. The purpose of this paper is to review the state of the art, link the theoretical concepts to practical cases and to indicate where further research is required to take next steps towards a marketable product. First, we describe the capacitive link via a coupling model. Next, we highlight the recent progress in plate topologies. Additionally, the most common compensation networks, necessary for achieving efficient power transfer, are reviewed. Finally, we discuss power electronic converter types to generate the electric field.
Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of PV cells is much lower. Typically, the light intensity under artificial lighting conditions is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. In this context, the question arises whether thin film chalcogenide photovoltaic cells are suitable for indoor use.This paper contributes to answering that question by comparing the power output of different thin film chalcogenide solar cells with the classical crystalline silicon cell as reference. This comparison is made for typical artificial light sources, i.e. an LED lamp, a "warm" and a "cool" fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM 1.5 spectrum as reference. All light sources (including the outdoor spectrum) are scaled to an illumination of 500 lux to obtain a correct comparison. The best artificial light source for all cell types is the incandescent lamp which improves the performance of the cell up to a factor 3 compared with the AM 1.5 spectrum. One remarkable result is that a CdTe cell outperforms a CIGS cell with more than 33 % in an indoor artificial lighting environment (except with an incandescent light source).
In this paper, the use of a repeater element between the transmitter and the receiver of a capacitive wireless power transfer system for achieving larger transfer distances is analyzed. A network formalism is adopted and the performance described by using the three power gains usually adopted in the context of two-port active networks. The analytical expressions of the gains as function of the network elements are derived. Assuming that the parameters of the link are given and fixed, including the coupling factors between transmitter, repeater and receiver, the conditions for maximizing the different gains by acting on the network terminating impedances (i.e., load and internal source conductance) are determined. The analytical formulas are verified through circuital simulations.
Typical wireless power transfer (WPT) systems on the market charge only a single receiver at a time. However, it can be expected that the need will arise to charge multiple devices at once by a single transmitter. Unfortunately, adding extra receivers influences the system efficiency. By impedance matching, the loads of the system can be adjusted to maximize the efficiency, regardless of the number of receivers. In this work, we present the analytical solution for achieving maximum system efficiency with any number of receivers for capacitive WPT. Among others, we determine the optimal loads and the maximum system efficiency. We express the efficiency as a function of a single variable, the system kQ-product and demonstrate that load capacitors can be inserted to compensate for any cross-coupling between the receivers.
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